Making the structure of 'fire ice' with nanoparticles

Cage structures made with nanoparticles could be a route toward making organized nanostructures with mixed materials, and researchers at the University of Michigan have shown how to achieve this through computer simulations.

The finding could open new avenues for photonic materials that manipulate light in ways that natural crystals can’t. It also showcased an unusual effect that the team is calling entropy compartmentalization.

“We are developing new ways to structure matter across scales, discovering the possibilities and what forces we can use,” said Sharon Glotzer, the Anthony C. Lembke Department Chair of Chemical Engineering, who led the study published in Nature Chemistry ("Entropy compartmentalization stabilizes open host–guest colloidal Clathrates"). “Entropic forces can stabilize even more complex crystals than we thought.”

The cages of the host network of bipyramid particles are shown in blue on the left side, becoming increasingly transparent toward the right. The red bipyramid particles are guest particles, trapped in the cages of the clathrate
While entropy is often explained as disorder in a system, it more accurately reflects the system’s tendency to maximize its possible states. Often, this ends up as disorder in the colloquial sense. Oxygen molecules don’t huddle together in a corner—they spread out to fill a room. But if you put them in the right size box, they will naturally order themselves into a recognizable structure.
Nanoparticles do the same thing. Previously, Glotzer’s team had shown that bipyramid particles—like two short, three-sided pyramids stuck together at their bases—will form structures resembling that of fire ice if you put them into a sufficiently small box. Fire ice is made of water molecules that form cages around methane, and it can burn and melt at the same time. This substance is found in abundance under the ocean floor and is an example of a clathrate. Clathrate structures are under investigation for a range of applications, such as trapping and removing carbon dioxide from the atmosphere.

The full particle shapes are shown on the left, with the blue particles forming the cage network structure and the red acting as guests. On the right, the cages are traced out with blue dots at each point or truncated point on the particles and gray lines connecting them. (Image: Sangmin Lee, Glotzer Group)
Unlike water clathrates, earlier nanoparticle clathrate structures had no gaps to fill with other materials that might provide new and interesting possibilities for altering the structure’s properties. The team wanted to change that.
“This time, we investigated what happens if we change the shape of the particle. We reasoned that if we truncate the particle a little, it would create space in the cage made by the bipyramid particles,” said Sangmin Lee, a recent doctoral graduate in chemical engineering and first author of the paper.
He took the three central corners off each bipyramid and discovered the sweet spot where spaces appeared in the structure but the sides of the pyramids were still intact enough that they didn’t start organizing in a different way. The spaces filled in with more truncated bipyramids when they were the only particle in the system. When a second shape was added, that shape became the trapped guest particle.

Glotzer has ideas for how to create selectively sticky sides that would enable different materials to act as cage and guest particles, but in this case, there was no glue holding the bipyramids together. Instead, the structure was completely stabilized by entropy.

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